1. Field of the Invention
The present invention relates to photolithography, and more particularly to an apparatus and method for exposing lithographic features on a semiconductor wafer. Specifically, the present invention defines a method and apparatus for adjusting a light polarizer in combination with a photo mask (reticle) having a built-in polarizer, to regulate the transmission intensity of the light about an exposed pattern on a semiconductor substrate.
2. Description of Related Art
Photolithography techniques are commonly used in industry to produce integrated semiconductor circuits. The resolution of this process governs the ability to develop smaller and finer reduced images. Typically, in photolithography, a substrate is coated with a photo-resist, imaged with the desired circuit pattern, developed, and subjected to an etching process in the exposed areas. The photo-resist is applied as a light sensitive polymer that is subsequently used to protect selected areas during chemical treatment. The photolithography process generates a product (photo mask) that essentially becomes the primary tool in transferring an image onto a semiconductor substrate.
Photolithography demagnifies objects in order to produce images on the photo-resist layer of the semiconductor surface. Objects are reduced and imaged on the substrate in order to develop a pattern of microelectronic circuits on the photo-resist.
Mathematically, the resolution of this projected image is proportional to an exposure wavelength, .lambda., and inversely proportional to the numerical aperture, NA, of the optical system. The resolution may be expressed as: EQU R=K.lambda./(NA)
where, PA1 where,
R=resolution PA2 K=process constant PA2 .lambda.=wavelength of exposure light PA2 NA=numerical aperture of exposure system PA2 .DELTA..phi. is the phase-shift in radians; PA2 n is the index of refraction of the transmission material; PA2 d is the thickness of the material in meters; and PA2 .lambda. is the wavelength of the exposing light in meters.
As miniaturization continues, efforts have been made to reduce the process constant, shorten the exposure wavelength, or increase the numerical aperture, in order to print finer, more resolute images. Nevertheless, during this miniaturization process, the demagnified objects projected on the photo-resist layer of the semiconductor surface must maintain the proper shape, light intensity profile, and contrast, in order to meet the minimum requirements for image transfer to the semiconductor substrate during production.
One method taught to enhance resolution is a phase shifting method, in which the improvement of the resolution and the increase in the depth of focus are intended by introducing a phase shift between light passing through adjacent transparent portions on a photo mask. In "IMPROVING RESOLUTION IN PHOTOLITHOGRAPHY WITH A PHASE-SHIFTING MASK", IEEE Transaction on Electronic Devices, Vol. ED-29, December 1982, by Levenson, phase-shifting masks were originally proposed to improve the pattern image resolution and tolerance controlled by creating out-of-phase destructive interference at the dark-light boundaries of adjacent apertures.
Generally, the phase shifted light will cause constructive and destructive interference patterns. This interference, in turn, directly affects the intensity of the transmitted light. A photo-resist and a spin-on-glass (SOG) material are each known as a material which forms the phase shifting properties, with the SOG material being structurally more robust.
In U.S. Pat. No. 5,656,397 issued to Imai et al., on Aug. 12, 1997, entitled, "MASK HAVING A PHASE SHIFTER AND METHOD OF MANUFACTURING THE SAME", a method of manufacturing a photo mask having a phase shifter which can control the phase of exposure light was introduced. In the Imai invention, a phase shifter is formed on a glass plate substrate such that fluctuations in the film thickness of the phase shifter are minimized in the areas where the dimension or density of the light shielding pattern is different. The effect is to alter the phase of light passing through a first transparent area in the mask and the phase of light passing through a second transparent area in the mask, such that the joined light is canceled from each other (destructive interference) thereby greatly reducing the light intensity. Thus, phase information and amplitude information are employed to generate enhanced image characteristics.
A phase-shift is developed when light is transmitted through a transparent material exhibiting a temporal phase-shifting that can be represented by: EQU .DELTA..phi.=2.pi.(n-1)d/.lambda.
The technique of adding phase shifted information to the amplitude information of the transmitted light, although employed to further enhance the resolution of reduced images, remains limited in the resultant image performance. Consequently, the art has continually progressed to eventually incorporating a third form of light information, polarization, as a means to establish satisfactory high contrast images.
In U.S. Pat. No. 5,541,026 issued to Matsumoto on Jul. 30, 1996, entitled, "EXPOSURE APPARATUS AND PHOTO MASK", an exposure apparatus is taught for transferring the pattern contained in a photo mask onto a substrate by illuminating the photo mask using a projection optical system and a polarizer at the position of the incident pupil of the optical system. A polarizer is added to each light transmission portion in the photo mask such that the polarizers, in combination, produce polarized beam conditions perpendicular to each other. Although the Matsumoto invention teaches the effects of using polarization to change the frequency and distribution of the diffraction pattern in order to increase image resolution, this must be predetermined in advance. Thus, there is no provision for varying the polarization in-situ, i.e., adjusting the light intensity during light transmission or exposure such that the polarizers are not always situated to be perpendicular to each other.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus and method for enhancing the contrast of a lithographic image.
It is another object of the present invention to provide an apparatus and method for adjusting the light intensity during the exposure of a photolithographic image.
A further object of the invention is to provide an apparatus and method for developing an in-situ adjustable photolithographic device.
It is yet another object of the present invention to provide an apparatus and method for forming patterns on a semiconductor substrate using a photo mask polarizer and a separate rotating polarizer.
Another object of the present invention is to replace several attenuated masks, e.g., 0.5%, 1%, and 6% reticles with one attenuated reticle that can be adjusted in-situ.
Still other advantages of the invention will in part be obvious and will in part be apparent from the specification.